Epoxy resins are used for adhesives, molding materials, coating materials, photoresist materials, developer materials, and the like. In addition, epoxy resins can provide cured products that are excellent in terms of heat resistance, moisture resistance, and the like and hence are widely used for, for example, semiconductor sealing materials and insulating materials for printed wiring boards in electric and electronic fields.
Among these various applications, in the field of printed wiring boards, with the trend toward electronic devices having a smaller size and higher performance, there is a strong tendency of narrowing the wiring pitch of semiconductor devices to achieve a higher density. In response to this tendency, a semiconductor mounting method that is a flip chip bonding method in which a semiconductor device is bonded to a substrate with solder balls is widely used. This flip chip bonding method is a so-called reflowing semiconductor mounting method in which solder balls are placed between a wiring board and a semiconductor and the entire structure is heated to achieve fusion bonding. Accordingly, the wiring board itself is exposed to a high-temperature environment during solder reflowing; the wiring board is thermally contracted to cause a large stress in the solder balls that bond together the wiring board and the semiconductor, resulting in poor connection of the wiring in some cases. For this reason, there is a demand for an insulating material having a low thermal expansion coefficient for printed wiring boards.
In addition, in recent years, due to laws, regulations, and the like relating to the environmental issues, lead-free high-melting-point solders have been mainly used and the reflowing temperature has become high. As a result, poor connection due to warpage of printed wiring boards caused by change in the heat resistance of insulating materials during reflowing has become severe. Accordingly, there is a demand for materials that are less changed in terms of physical properties during reflowing.
In order to satisfy such a demand, for example, a thermosetting resin composition containing, as a main component, a naphthol-novolac epoxy resin obtained by the reaction between naphthol, formaldehyde, and epichlorohydrin has been proposed as a composition that achieves the technical objects such as low thermal expansion (refer to Patent Literature 1 below).
Compared with general phenol-novolac epoxy resins, the above-described naphthol-novolac epoxy resins allow, due to their rigid skeletons, an improvement in the thermal expansion coefficients of the resultant cured products. However, this improvement does not sufficiently satisfy the required level in recent years. In addition, the cured products are considerably changed in terms of heat resistance due to thermal history. Accordingly, in the printed-wiring-board applications, the heat resistance is considerably changed due to reflowing, which tends to result in the above-described poor connection in printed wiring boards.
There is another known naphthol-novolac epoxy resin, for example, an epoxy resin produced by polycondensation between α-naphthol, β-naphthol, and aldehyde (refer to Patent Literature 2 below). However, such an epoxy resin has a high content of a dimer of β-naphthol and hence the resin has high crystallinity and low solvent solubility and is not sufficiently cured during the curing reaction. Accordingly, the resultant cured products are also considerably changed in terms of heat resistance due to thermal history.